Aesthetic and biocompatibility considerations have greatly increased the demand for metal-free dental restorations in clinical dentistry, leading to great demand to replace the once-common metal or metal-backed ceramic fillings, crowns, veneers, bridges, posts, and other dental prosthetics and restorations with ceramic (also known as full-porcelain) versions of those restorations. However, traditional glass-ceramic and aluminum oxide ceramic restorations display a brittleness, high propensity for crack propagation, low tensile strength, and poor wear resistance that limits their use or longevity in many applications for dental restorations.
Zirconia-based technologies, such as zirconium-oxide materials, have greatly overcome the poor performance properties of traditional ceramic restorations with their high strength and comparatively higher fracture toughness, and may be used in endodontic posts, implants, and implant abutments, orthodontic brackets, cores for crowns, and fixed partial denture prosthesis frameworks, and other dental restorations. Further, zirconia provides the metal-free, aesthetic characteristics requested by patients, and its hard and dense surface is ideal for resisting wear damage making zirconia an attractive material for single tooth dental restorations.
However, while zirconia dental restoration materials (including those sold under the LAVA, CERCON, and PROCERA trademarks) show a marked improvement in wear and strength properties in dental applications over traditional ceramics, these materials have proven to be challenging to adhere to dentin (whether etched or unetched), enamel, resins, and other materials using traditional dental materials and techniques. The questions of how to prepare the internal surfaces of restorations and restoration cites, as well as what proper adhesive protocols will result in clinically optimal results are current challenges of zirconia bonding because the clinically established protocol of etching (i.e., with hydrogen fluoride) and silanation, effective for other glass ceramic materials, does not yield sufficient strength when applied in zirconia bonding. Thus the acid-resistant, silica free surface of zirconia creates difficulty in establishing a strong and stable bond between the zirconia internal surface and tooth structure.
Previous attempts to improve the adhesion between resin materials and zirconia include U.S. Pat. No. 6,939,901 to Nakatsuka et al. This system utilizes a two-part adhesive system comprising a first polymerizable monomer containing an acidic group and a second polymerizable monomer having a general formula of
with R1 being hydrogen or a methyl group; R2 being a halogen; hydroxyl group, mercapto group, or —O—R3—OH group; R3 being an alkene group having 6 to 25 carbons; and Y being oxygen or sulfur. Similarly, U.S. Pat. No. 6,512,068 to Nakatsuka utilizes an adhesive system comprising a water-insoluble acid monomer having an alkene group of 8-25 carbons atoms, an alkyl group having 8-25 carbon atoms, an aromatic group, and an acid group selected from phosphoric acid, thiophosphoric acid, carboxylic acid, sulfonic acid, or another similar acid group, and a polymerizable unsaturated group selected from an acryloyl group, a methacryloyl group, a vinyl group, a styrene group; wherein the water-insoluble acid is present as a salt by combining the water-insoluble acid with a base in water to create a composition with a pH of 1.0 to 6.0. However, tests of commercial embodiments of these systems reveal bonding strengths between zirconia materials and other dental materials that would preferably be higher in clinical applications. Therefore an improved system for bonding, including greater bond strength and durability of bond between both resin materials and zirconia, as well as bonding between etched and un-etched dentin and resin materials in clinical applications would be appreciated in the art.